Polypropylene resins such as propylene homopolymers and ethylene-propylene copolymers are widely used in automobiles, household appliances, construction materials, furniture, packing containers, toys, miscellaneous daily goods and the like. However, since polypropylene resins are inferior to polyethylenes, polyethylene telephthalates, polycarbonates, polystyrenes and the like in transparency, incorporation of various nucleating agents and transparentization agents such as metal salts of benzoic acid, metal salts of aromatic phosphate esters, metal salts of alicyclic compounds, dibenzylidene sorbitols, and amide compounds has been suggested since before and put into practical use.
Among these, dibenzylidene sorbitols are excellent in an effect of improving transparency, but they have defects that they have strong odor, they have a high melting point and thus are difficult to be dispersed homogeneously in resins at a processing temperature, and the like. Although homogeneous dispersion is possible by adjusting the processing temperature to the melting point or more, processing at the melting point or more is not preferable for a compound represented by the general formula (I) which is used in the present invention since it has a high melting point of 260° C. or more and thus processing at the melting point or more causes significant deterioration of polypropylene. In order to improve dispersibility in resins at the melting point or less, various treatments such as decreasing of the particle diameter by pulverization and decreasing of the melting point by melt-mixing with other component in advance have been suggested.
For example, Patent Literature 1 suggests expressing performances by forming a dibenzylidene sorbitol into a powder having d97 of 30 μm or less. Although this dibenzylidene sorbitol is excellent in an effect of improving transparency, it has problems that the flowability is decreased by forming into a micropowder, thereby handling properties such as measuring and charging are decreased, and that expected dispersibility cannot be obtained due to blocking, and the like. Furthermore, it is actually necessary to prevent secondary aggregation of the micropulverized product by using an inorganic substance, a surface treating agent or the like, and thus the production cost is increased and components that are unnecessary for the resin are mixed. Since mixing of unnecessary components is associated with unintended change in the physical properties of the resin and leads to decrease in the quality during recycling and limitation to the usage, micropulverization is not necessarily a satisfying solution means.
Patent Literature 2 suggests improving performances by using a mixture of dibenzylidene sorbitol compounds having different structures. Since melting point depression occurs by forming a mixture of substances having analogous structures, dispersibility in resins is improved. However, since a transparentization agent having other structure is daringly added to a compound that is specifically excellent in effects among dibenzylidene sorbitols, it is not a sufficient countermeasure due to that the transparentization effect is decreased, and the like.
Furthermore, since polypropylene alone is deteriorated at the processing temperature, it is essential to incorporate additive components such as phenol antioxidants and phosphorus antioxidants, and these additive components have been conventionally incorporated in powders or pellets of polypropylene together with a nucleating agent, a transparentization agent and the like. As is described in Patent Literature 1 as Comparative Examples, even these other additive components are incorporated in a resin together with a dibenzylidene sorbitol and mixed by a Henschel mixer or the like, the dispersibility of the dibenzylidene sorbitol could not be improved.